FIG. 4 is a heat transfer surface arrangement diagram illustrating one example of a conventional exhaust gas boiler. In this figure, reference numeral 1 represents an exhaust gas flow path; 2, a high-pressure superheater; 3, a high-pressure evaporator; 5, a high-pressure economizer; 6, a low-pressure evaporator; 7, a low-pressure economizer; 8, a high-pressure steam drum; 13, a denitrator; 15, an ammonia injection nozzle; 16, a high-pressure feed water pump; 17, a low-pressure feed water pump; 18, a low-pressure steam drum; and 19, a stack. The fluid temperatures inside the various heat transfer surfaces of this exhaust gas boiler and the gas temperatures outside them are distributed as shown by solid lines in FIG. 5.
In the above-described conventional exhaust gas boiler, if sulfur oxides are contained in the combustion gas, acid ammonium sulfate may be deposited on the low-temperature heating tubes disposed in a gas temperature region located downstream of the denitrator 13 and having temperatures of 200.degree. C. or below. If so, the increase in gas-side draft loss caused by corrosion of the tubes or clogging of the tubes (especially of the finned tubes) presents a problem. For this reason, heat transfer surfaces cannot be disposed in low-temperature regions (i.e., regions B and C in FIG. 5). Consequently, the heat transfer surface arrangement of FIG. 4 which permits highly efficient heat recovery cannot be employed, and a simpler heat transfer surface arrangement as illustrated in FIG. 6 must be employed. Thus, the outlet gas temperature can only be reduced to a level of as high as 200.degree. C.
Even where a clean gas such as LNG is burned, the surplus ammonia level at the outlet of the denitrator usually needs to be reduced to 10 ppm or less. For this reason, it has been difficult to enhance the efficiency of the denitrator.
As a countermeasure to the above-described deposition of acid ammonium sulfate, there is known a method for suppressing the growth of acid ammonium sulfate in which part of the high-temperature gas on the upstream side of the denitrator is suitably supplied to the heat transfer surfaces located downstream of the denitrator and subject to the deposition of acid ammonium sulfate so as to raise the gas temperature around the heating tubes. However, this method is impractical because a complicated arrangement of high-temperature ducts is required and such a high effect as enables the removal of already deposited acid ammonium sulfate cannot be expected. Moreover, if the high-temperature gas is always made to flow, heat absorption in the upstream high-temperature section is decreased and heat absorption in the entire exhaust gas boiler is also decreased. Thus, this method cannot be regarded as suitable even from the viewpoint of plant efficiency.
Furthermore, in the case of clean gases, there has been proposed a method for removing the surplus ammonia by adsorption at the outlet of the denitrator. However, this method has the disadvantage, for example, of increasing the size of the equipment.